1. Field of the Invention
This invention relates generally to apparatus for studying the physical properties of particles carried in suspension and more particularly to such apparatus and fluid circulating system therefor for use in connection with analysis of particles having considerable size and density.
2. Description of the Prior Art
There is a need in many fields concerned with particulate substances and aggregates to analyze such particles and determine various characteristics thereof. For example, analysis of such particles, often called "large particles", is required in fields concerned with granulated metals, sands, powders and the like.
One manner of performing analyses on such large particles is with the use of sieve shakers which include meshed screens or sieves into which the particulate material or aggregate is deposited. Analysis of large particles with the use of such sieve shakers gives rise to many disadvantages such as excessive time required to perform the analyses, the need to calibrate individual sieves due to wear and the need to clean the sieves before each re-use thereof.
Attempts have been made to provide electronic particle study apparatus suitable for performing analyses of such large particles while obviating the disadvantages of sieve shakers. One such electronic particle study apparatus is disclosed in U.S. Pat. No. 3,688,191 utilizing structure termed "a Coulter scanning aperture means" as defined in U.S. Pat. No. 2,656,508 (the mark "Coulter" is a Registered Trademark, Registration No. 995,825, of Coulter Electronics, Inc. of Hialeah, Florida). The structure of said U.S. Pat. No. 3,688,191 includes a pair of chambers separated by a Coulter scanning aperture means to perform the large particle analyses desired.
While the structure of U.S. pat. No. 3,686,191 improved upon the sieve shaker method of large particle analyses, other problems occurred in the use of such structure. The patented structure requires that the large particles to be analyzed be maintained in a homogeneous suspension which is difficult to achieve even with the use of an agitation device as disclosed in the patent. Further, the structure of the patented apparatus is such that particles which have been sensed by the Coulter scanning aperture means tend to accumulate and built up behind the aperture resulting in spurious signals being produced. Additionally, often it is desired to change the size of the aperture in such structure so that analyses of different sized particles may be made; the said patented apparatus does not provide convenient means for easily accomplishing removal of one aperture and assembling within the apparatus a different sized aperture.
It also is known that providing for straight line passage of the particles to be analyzed through the center of the aperture by use of a director member will improve the signals detected by a Coulter scanning aperture means. The ratio of the size of the aperture with the distance thereof to the end of the director is known preferably to be maintained constant for optimum results. The provision of such straight line passage of particles through the aperture can be achieved by hydrodynamic focusing thereof together with introduction of sheath flow liquid at the location where the particles are introduced into the aperture. The teaching of such principals can be found in the following patents and publications: U.S. Pat. Nos. 3,831,087, 3,810,010 and 3,793,587; German published patent application No. 2,050,847; P. Crosland-Taylor, et al., "An Electronic Blood-Cell Counting Machine", Blood, Vol. 13, No. 4, April, 1958, pp. 398-409; P. Crosland-Taylor, "A Device for Counting Small Particles suspended in a Fluid through a Tube", Nature, Vol. 171, No. 4340, January, 1953, pp. 37-38. Further, to avoid consuming large volumes of fluids which pass through the relatively large sized apertures used in analyzing large particles, it is known to provide for a closed system in which the fluid continuously is pumped back for recirculation through the system.
Although the various principals referred to have been acknowledged separately in the art as contributing to the achievement of a highly desirable particle analysis apparatus and system, these principals have not heretofore been included together in an advantageously cooperative manner to produce a commercially acceptable apparatus. The present invention employs these principals in a novel structure to provide a highly efficient and compact apparatus and system for analyzing large particles without the disadvantages of the prior art structures.